Experiments on single-phase flow through packed beds at 1.025 < D/d < 2.88 are conducted using water, where the packed beds have advantages such as high radial mixing intensity, intensified heat and mass transfer, safety and uniformity of temperature. The structure of slender packed beds has remarkable influence on the pressure drop characteristics. For packed beds at 1.03 < D/d < 1.98, the coefficients of the Ergun equation show strong dependence on the aspect ratios. With the aspect ratios increasing to D/d > 2, more than one stable configuration can be found at the same aspect ratio. Hence, the influence of assembly method and its corresponding structure on the pressure drop are investigated. It is found that a minor change on the local structure could lead to notable pressure drop difference with very same porosity value in packed beds. And the pressure drop increase is observed as well with increasing mean porosity in the experiment. Therefore, the empirical correlations in literature fail to satisfy the experimental results. Furthermore, with proper assembly method, a highly order structure of packed bed can be achieved, which shows good performance on the pressure drop reduction. These experimental results would be useful for the optimum design in industrial applications as well as understanding the transport phenomena in packed beds. (C) 2017 Elsevier Ltd. All rights reserved.